As turbocharged diesel engines have developed to higher brake mean effective pressure (BMEP) levels, with low levels of legislated emissions, it has become increasingly difficult to match the turbine and compressor in conventional single compressor and/or single turbine turbochargers, and achieve the desired level of performance. Emissions regulations enacted but not yet in force, may force extreme levels of Exhaust Gas Recirculation to control NOx and Particulate Filters to control soot and particulate emissions. Generally, the compressor must provide the level of mass flow that the engine requires at its maximum power, and this requirement operates to set the size of the compressor. In general, the compressor inducer throat area, i.e., the portion of the compressor that meets incoming air and that is characterized by design parameters that include the inducer diameter, blade inlet angle, and blade blockage, determines the compressor flow. The boost pressure needed to achieve a given level of mass flow is a function of the engine design and flow characteristics. The speed of the compressor is determined by its diameter and impeller blade backward curvature. The turbine must produce the power necessary to drive the compressor at the speed demanded by the compressor to reach the boost pressure and mass flow required by the engine. Thus there is always a compromise to achieve the turbine match.
Radial turbines operate best when the turbine blade tip speed divided by the isentropic spouting velocity (commonly referred to as U/Co) is approximately 0.7. Unfortunately, several design features of future engines make this difficult to achieve. The maximum corrected flow of the turbine is a function of its size and blade curvature. Ultra-high boost pressures reduce the required maximum turbine corrected flow, as does the inclusion of exhaust gas recirculation (EGR). EGR essentially reduces the fresh air volumetric efficiency of the engine, thus requiring higher boost pressures to pass the required fresh air. This in turn requires more turbine power which is achieved by increasing the backpressure on the engine.
Other devices that increase the backpressure on the turbine such as diesel particulate filters, all types of catalysts, or turbo-compound turbines also operate to reduce the required turbine corrected flow. As the pressure at which the turbine discharges to is raised, and the pressure ratio of the turbine is held constant to produce the required power, the inlet pressure of the turbine is significantly increased. This increase in turbine inlet pressure results in higher density exhaust gas, and thus lower corrected flow. As the power densities have increased and the aforementioned devices have become more and more common, the challenge to correctly match the turbine and compressor has increased.
Achieving good low engine speed performance requires that the turbine flow be reduced to generate good boost pressures with the minimum exhaust energy that is available. This has given rise to the use of variable geometry turbines. In such variable geometry turbines the turbine geometry is configured to be controlled to reduce the flow area of the turbine and generate more backpressure. This higher backpressure results in an increased expansion ratio for the turbine, which functions to create more turbine power.
When taken to extremes, such as that seen when accelerating the engine rapidly from idle, the turbine performance is quite poor. There are several causes for this. First, the wheel and turbine nozzle are operating at far off design, and the U/Co is not in the optimum operating zone. Second, the turbine flow area is substantially closed resulting in a high-pressure loss through the flow control device (such as an adjustable vaned nozzle cascade).
Conventional turbochargers would have to be configured having a large compressor with a very small turbine due to the aforementioned reasons. To counteract this, it is possible to use a high trim (large inducer size) compressor, with high backward curvature to increase the turbocharger speed to improve the turbine match. The turbine can also be configured having a low trim turbine to match the flow characteristic and force the diameter of the turbine as large as possible. However, these design alternatives may not be desirable from other points of view such as fatigue life, packaging, efficiency, inertia, etc.
In conclusion, the future highly rated, low emission turbo-diesel engine will require a fundamentally different concept of turbocharger design than is presently provided by conventional turbochargers comprising a single radial compressor driven by a single radial turbine.
To overcome the above-noted deficiency, different approaches have been taken that each involve using two turbochargers. The three most popular two-turbocharger configurations are commonly referred to as series, parallel/sequential, or staged. The common theme of all three concepts is that a small turbocharger (roughly half the size or smaller than a normal full range single turbocharger) is used at the low end of the speed range for best performance.
Enhanced transient performance is achieved by the initial use of a smaller turbocharger due to a number of reasons. First, the compressor is not operating near the surge line where efficiency is poor, but closer to the peak efficiency island. Second, the turbine is also much smaller and better matched for the low flows. This is true whether fixed geometry, wastegated and variable geometry turbines are used. For fixed geometry and wastegated turbines, the turbine housing A/r (which controls the flow characteristic) is closer to the optimum for efficiency. With a variable geometry turbine, the turbine size is reduced, and the nozzle setting becomes more open and reduces the flow loss through the vane cascade. Third, a smaller turbocharger has less rotating group inertia, thus less turbine power is consumed increasing the speed of the turbocharger and is applied to the compressor to generate boost pressure.
A second turbocharger matching problem is known to exist with a traditional single turbo approach, separate and apart from the turbine matching issues discussed above for these new engines. The improved engine responsiveness at low speed, combined with high power levels at full speed, has resulted in a compressor range problem. Utilizing state-of-the-art aerodynamic analysis to increase the flow range of the compressor has yielded impressive improvements. However they still fall short of engine manufacturer's expectations. The compressor surge line (a parameter in defining the compressor flow range) limits many engines' low speed torque.
This flow range issue has resulted in development work on variable geometry compressors as well as the use of two turbochargers as mentioned previously. While variable geometry compressors can improve the performance of the compressor, they add more complexity, moving parts, cost, and control elements to the engine. While multiple turbochargers in either staged, series, or parallel/sequential arrangement, can help improve engine performance at the low end of the speed range and have improved compressor range, they also add cost, complexity, weight, and packaging challenges to the engine.
It is, therefore, desirable that a turbocharger be constructed in a manner to provide a degree of turbocharger matching that enables the engine to produce the desired BMEP level and meet legislated emissions limits. It is desired that such a turbocharger be constructed in a matter that can permit retrofit or new application use with a minimum of ancillary modifications. It is further desired that such a turbocharger be constructed in a manner that is space efficient to promote efficient engine compartment packaging.